The present invention relates to a polymer whose solubility in water changes depending on even a very slight difference in the concentration of a salt contained in the water and more specifically to a salt-sensitive polymer which is soluble in a usual tap water but is insoluble in an aqueous solution containing not less than 0.5% by weight of a neutral inorganic salt comprising a monovalent ion such as NaCl, KCl or NaBr.
It has been known that when an inorganic salt which is an electrolyte is added in an increasing amount to an aqueous solution of a water-soluble polymer, the polymer is salted out to form a precipitate. On the other hand, it has also been known that salts are contained in human body fluids such as urine in an amount of at least about 0.5% by weight. It is supposed, therefore, that when a non-woven fabric or paper is bonded with a binder comprising a polymer which is soluble in tap water but insoluble in an aqueous solution containing salts in an amount of 0.5% by weight, the resulting product will have a sufficient strength when it is brought into contact with a body fluid, but the product will be easily dispersed in a flush toilet. Under these circumstances, investigations are made on the use of the salt-sensitive polymer as a binder for various products to be thrown into the flush toilet or the like.
Among the salt-sensitive polymers, anionic polymers such as sodium salts of polyacrylic acid and carboxymethyl cellulose are less sensitive to an increase in the concentration of an electrolyte in an aqueous solution. Such anionic polymers become insoluble in, for instance, an aqueous solution of common salt when the common salt concentration is increased to as high as 4 to 5% by weight or above. Although Japanese Patent Unexamined Publication (hereinafter referred to as "J. P. KOKAI") No. Sho 50-52371 discloses an alkali cellulose ether sulfate, the difference between the solubility of this sulfate in tap water and that in 0.5 wt. % aqueous common salt solution is not clearly described in this patent. Moreover, since the alkali cellulose ether sulfate is prepared by sulfating a cellulose derivative such as ethyl cellulose, the polymer used as the starting material is relatively expensive.
As for a salt-sensitive polymer for solving the foregoing problems, J. P. KOKAI No. Sho 63-139906 discloses a partially neutralized copolymer of (meth)acrylic acid and a (meth)acrylic ester as the polymer soluble in tap water but insoluble in an aqueous solution of common salt having a concentration of not less than 0.5% by weight. However, such a partially neutralized copolymer has a high adhesion. J. P. KOKAI No. Hei 3-174417 discloses a partially neutralized copolymer of (meth)acrylic acid and an aryl or cycloalkyl (meth)acrylate; or a partially neutralized copolymer of (meth)acrylic acid and a styrene derivative which copolymers have no problem of the adhesion. However, non-woven fabrics or papers prepared by using such a polymer as the binder do not have satisfactory strength and permeability to the body fluid practically demanded when they are wetted with the body fluid.